can be found spread around in The Underbelly (The Sewers of Dalaran, Northrend). The best time to look for them is when there are few people online, if you don't find any - have a look some other time.
The Underbelly Elixir gives you one of three buffs when you drink it; you either see everyone as a mage, you turn into a tuskar or you turn into a for 10min (this is the one you want).
Once you have some Underbelly Elixir you want to go to your factions portal-room. Now drink the Elixir and hope for the wasp buff - once you have it you quickly need to use the portal to travel to your destination (SW in my case).
Tada! You are now flying in the old world (at least for 10min)!
In theory this should be usable with a mage portal or a warlock/meetingstone summon, it might even work in BGs/Instances - I have however not had the chance to try this yet.
This elixir also gives you the ability to fly in Northrend before level 77.